1. Field of the Invention
The present invention relates to a method of manufacturing steel sheets having excellent deep-drawability which sheets may be suitably used in manufacturing automobile bodies. Specifically, the present invention relates to a method of manufacturing hot-rolled steel sheets having excellent deep-drawability, as well as to a method of manufacturing surface-treated steel sheets.
2. Description of the Background Art
When steel sheets are prepared for deep drawing so that they may be used in manufacturing automobile bodies, they are required to have high Lankford values (r-values) and a high ductility (El: Elongation value). Such a steel sheet has generally been prepared as cold-rolled steel sheet manufactured by effecting hot rolling which is terminated at temperatures not lower than the Ar3 transformation point, subsequently obtaining the final thickness by cold rolling, and thereafter effecting recrystallization annealing. In recent years, however, in view of reducing production costs, there have been increasing demands for the substitution of members, which have hitherto been formed of cold-rolled steel sheet, with those formed of hot-rolled steel sheet.
In regard to hot-rolled steel sheet for use in working, it has hitherto been prepared in such a manner that, in order to assure satisfactory working properties, in particular ductility, rolling is terminated at temperatures not lower than the Ar3 transformation point so as to avoid formation of nonrecrystallized ferrite. However, since random orientation usually occurs in the texture during the .gamma. to .alpha. transformation, a hot-rolled steel sheet has considerably poor deep-drawability when compared with cold-rolled steel sheet. Hitherto, the r-value of hot-rolled steel sheet has ranged from 0.8 to 0.9 at most.
Recently, however, several methods of obtaining hot-rolled steel sheet excellent in deep-drawability have been proposed, in which no cold rolling is required. For instance, Japanese Pat. Laid-Open No. 226149/1984 discloses an example of a hot-rolled steel sheet having an r-value of 1.21 which is manufactured by subjecting low-carbon Al killed steel containing C: 0.002%, Si: 0.02%, Mn: 0.23%, P: 0.009%, S: 0.008%, Al: 0.025%, N: 0 0021%, and Ti: 0.10% to rolling at a reduction of 76% and at temperatures ranging from 500.degree. to 900.degree. C. while a lubricant is supplied, so as to obtain a steel strip having a thickness of 1.6 mm. In this method, however, because strong lubricated rolling must be effected during hot rolling, this inevitably involves some operational problems such as the risk of slipping occurring in the steel blank during rolling. Japanese Patent Laid-Open No. 192539/1987 discloses an example of a hot-rolled steel sheet having an r-value of 1.41 which is manufactured by subjecting low-carbon Al killed steel containing C: 0.008%, Si: 0.04%, Mn: 1.53%, P: 0.015%, S: 0.004%, Ti: 0.068%, and Nb: 0.024% to rolling at a reduction of 92% and at temperatures ranging from the Ar3 transformation point to the Ar3 transformation point +150.degree. C. In this method, however, because hot rolling is terminated at a temperature within the .gamma.-phase range, and the transformed tissue resulting from the subsequent .gamma. to .alpha. transformation is utilized, this inevitably has a preferred orientation of {112}. As a result, the value of .DELTA.r that is indicative of planer anisotropy of the r-value becomes so great that .DELTA.r=-1.2. This is detrimental in practice.
In order to insure excellent deep-drawability, a method must achieve the relationship of r.gtoreq.1.4 at least, without involving operational problems in conducting hot rolling, and without causing anisotropy.
In regard to a steel sheet which is prepared for use in manufacturing automobile bodies, there have recently been increasing demands for a surface-treated steel sheet having surfaces which have been subjected to various kinds of surface treatments. Among various types of surface-treated steel sheets, one of the more superior is the hot dip galvanized sheet because this is advantageous in both production cost and its properties.
A hot dip galvanized steel sheet is required to possess various properties. One of the most important requirements is excellent corrosion resistance, while deep-drawability is another important requirement. Since outside or inside panels of automobiles are usually formed by strong press working, it must be prepared as a galvanized sheet which possesses both a high Lankford value (r-value) and a high level of elongation.
A method of manufacturing such a galvanized sheet possessing excellent deep-drawability is disclosed in, for instance, Japanese Patent Laid-Open No. 29555/1982. This patent publication proposes the art of attaining properties of the order of r=2.0 and El=49% by subjecting a steel containing C: 0.006 wt % ("wt %" will hereinafter be abbreviated to "%"), N: 0.0045%, Si: 0.008%, and Nb: 0.043% to hot rolling, pickling and cold rolling, and further subjecting the steel to recrystallization annealing and plating in a continuous galvanizing line. Japanese Patent Laid-Open No. 74231/1984 discloses the art of attaining properties of the order of r=2.1 and El=51% by subjecting a steel containing C: 0.003%, N: 0.005%, Si: 0.010%, Ti: 0.012%, and Nb: 0.007% to hot rolling, pickling and cold rolling, and further subjecting the steel to recrystallization annealing and plating in a continuous galvanizing line.
Although each of these methods is successful in manufacturing a galvanized sheet possessing excellent deep-drawability, a long series of processes has to be conducted before the final product is obtained. This means that great amounts of energy, labor and time must be consumed in order to manufacture such galvanized sheet.